(1) Field of the Invention
Expanded graphite is provided in the present invention. The present invention relates in part to polymer-expanded graphite composites. The graphite platelets are preferably reduced in size to less than about 200 microns. The invention also relates to expanded graphite used for fuel cells, for battery anodes and for catalytic converters. The graphite is preferably expanded using microwave or radiofrequency wave heating.
(2) Description of Related Art
Graphite is a well known material occurring in natural and synthetic form and is well described in the literature. Illustrative of this art is a monograph by Michel A. Boucher, Canadian Minerals Yearbook 24.1-24.9(1994).
Nanocomposites composed of polymer matrices with reinforcements of less than 100 nm in size, are being considered for applications such as interior and exterior accessories for automobiles, structural components for portable electronic devices, and films for food packaging (Giannelis, E. P., Appl. Organometallic Chem., Vol. 12, pp. 675 (1998); and Pinnavaia, T. J. et al., Polymer Clay Nanocomposites. John Wiley & Sons, Chichester, England (2000)). While most nanocomposite research has focused on exfoliated clay platelets, the same nanoreinforcement concept can be applied to another layered material, graphite, to produce nanoplatelets and nanocomposites (Pan, Y. X., et al., J. Polym. Sci., Part B: Polym. Phy., Vol. 38, pp. 1626 (2000); and Chen, G. H., et al., J. Appl. Polym. Sci. Vol. 82, pp. 2506 (2001)). Graphite is the stiffest material found in nature (Young's Modulus=1060 MPa), having a modulus several times that of clay, but also with excellent electrical and thermal conductivity.
A useful form of graphite is expanded graphite which has been known for years. The first patents related to this topic appeared as early as 1910 (U.S. Pat. Nos. 1,137,373 and 1,191,383). Since then, numerous patents related to the methods and resulting expanded graphites have been issued. For example, many patents have been issued related to the expansion process (U.S. Pat. Nos. 4,915,925 and 6,149,972), expanded graphite-polymer composites (U.S. Pat. Nos. 4,530,949, 4,704,231, 4,946,892, 5,582,781, 4,091,083 and 5,846,459), flexible graphite sheet and its fabrication process by compressing expanded graphite (U.S. Pat. Nos. 3,404,061, 4,244,934, 4,888,242, 4,961,988, 5,149,518, 5,294,300, 5,582,811, 5,981,072 and 6,143,218), and flexible graphite sheet for fuel cell elements (U.S. Pat. Nos. 5,885,728 and 6,060,189). Also there are patents relating to grinding/pulverization methods for expanded graphite to produce fine graphite flakes (U.S. Pat. Nos. 6,287,694, 5,330,680 and 5,186,919). All of these patents use a heat treatment, typically in the range of 600° C. to 1200° C., as the expansion method for graphite. The heating by direct application of heat generally requires a significant amount of energy, especially in the case of large-scale production. RF or microwave expansion method can heat more material in less time at lower cost. U.S. Pat. No. 6,306,264 discusses microwave as one of the expansion methods for SO3 intercalated graphite.
U.S. Pat. Nos. 5,019,446 and 4,987,175 describe graphite flake reinforced polymer composites and the fabrication method. These patents did not specify the methods to produce thin, small graphite flakes. The thickness (less than 100 nm) and aspect ratio (more than 100) of the graphite reinforcement was described.
Many patents have been issued related to anode materials for lithium-ion or lithium-polymer batteries (U.S. Pat. Nos. 5,344,726, 5,522,127, 5,591,547, 5,672,446, 5,756,062, and 6,136,474). Among these materials, one of the most widely investigated and used is graphite flakes with appropriate size, typically 2 to 50 μm, with less oxygen-containing functional groups at the edges. Most of the patents described graphite flakes made by carbonization of precursor material, such as petroleum coke or coal-tar pitch, followed by graphitization process.